1. Technical Field
The present invention relates in general to an improved digital storage system and in particular to a method and system for improving the integrity of read/write operations within a head disk assembly (HDA). More particularly, the present invention relates to a method for providing off-track recovery boundaries for magnetoresistive (MR) and giant magnetoresistive (GMR) heads. Still more particularly, the present invention relates to a method for predetermining an off-track recovery boundary for an HDA that may be utilized following a failed read attempt as the limiting parameter during remedial off-track recovery attempts, thus ensuring the integrity of the read operation.
2. Description of the Related Art
Generally, a digital data storage system consists of one or more storage devices that store data on storage media such as magnetic or optical data storage disks. In magnetic disk storage systems, an HDA includes one or more hard disk drives (HDDs) and a hard disk drive (HDD) controller to manage local operations concerning the disks. HDDs are rigid platters, typically made of aluminum alloy or a mixture of glass and ceramic, covered with a magnetic coating. Typically, two or three platters are stacked vertically on a common spindle that is turned by a disk drive motor at speeds often exceeding ten thousand revolutions per minute (rpm).
The only other moving part within a typical HDA is a head positioning system. The head positioning system includes a recording head associated with each side of each platter. In most modern drives, the recording heads are mounted at the end of small ceramic sliders which xe2x80x9cflyxe2x80x9d just above or below the platter""s surface, supported by an air bearing surface that are self-pressurized by the airflows generated by the rapidly spinning disk. Each head is connected to a flexible actuator arm apparatus which supports the entire head flying unit. More than one of such arms may be utilized together to form a single armature unit.
Each head scans the hard disk platter surface during a xe2x80x9creadxe2x80x9d or xe2x80x9cwritexe2x80x9d operation. The head/arm assembly is moved utilizing an actuator which is often a Voice Coil Actuator (VCA) driven by a servo voice coil motor (VCM). The stator of the VCM is mounted to a base plate or casting on which is mounted a spindle supporting the disks. The base casting is in turn mounted to a frame via a compliant suspension. When current is fed to the motor, the VCM develops force or torque which is substantially proportional to the applied current. The arm acceleration is therefore substantially proportional to the magnitude of the current. As the head nears the desired track, a reverse polarity signal is applied to the actuator, causing the signal to act as a brake, and ideally causing the recording head to stop directly over the desired track.
HDAs continue to be the primary, high performance storage technology in terms of bit rate transfer. The success of HDAs in this respect, originates from an ever increasing demand for storage capacity coupled with a consistent reduction in price per stored data unit (mega- or giga-bytes). Areal density (expressed as billions of bits per square inch of disk surface area) is the product of linear density (bits of data per inch of track) multiplied by track density (tracks per inch or xe2x80x9cTPIxe2x80x9d). New technologies aimed at improving recording head efficiency are required to accommodate higher areal densities. Among the most significant of such advances have been the development of the magnetoresistive (MR) head, the extended magnetoresistive (MRx) head, and the giant magnetoresistive (GMR) head technologies.
In addition to improved head design, modern HDA throughput and storage capacity have been substantially enhanced by improvement in actuator design which has resulted in increased head placement precision and speed. The more precisely the actuator can place the recording head, the greater the amount of data that can be packed onto a given area of disk surface (often referred to as areal density). To meet greater performance demands, the bandwidth of the HDA system must increase (its response time to head-position changes must decrease). This demand for increased servo bandwidth has resulted in ever faster and more compact HDA assemblies. As utilized herein, xe2x80x9coff-track recoveryxe2x80x9d refers to a data recovery attempt within a HDA in response to a failed read attempt. During an off-track recovery procedure, the actuator re-positions the head from its original read position (ostensibly the track center) on the assumption that the desired data field is located somewhat off-center with respect to the track center.
During operation of a HDA unit, it is important that the head remain positioned over the center of the track on which it is writing or reading data. This has become increasingly difficult as the tracks have become smaller in width and are spaced closer together in order to increase the overall data density of the disk. The head may be off-center due to variations in thermal expansion of the different parts of the HDA, head positioning actuator inaccuracies, imperfect axial rotation of the spindle motor, or the like.
One solution to the head mis-positioning is to move the head in incremental steps across the track while attempting to determine the actual track center. This method is described in further detail with reference to U.S. Pat. No. 4,485,418 issued Nov. 27, 1984; IBM Technical Disclosure Bulletin, Vol. 35, No. 4B, p.303, September 1992; IBM Technical Disclosure Bulletin, Vol. 29, No. 2, p.586, July 1986.
Incomplete erasure of a previously recorded data track presents the potential for reading old data. When a new data field is written on a track, it overwrites the previous old data field. However, if the head is not properly centered on the track during the write operation, then a slice of the old data may remain at the edge of the new data field. This may cause an undetected false data read error when a read is subsequently attempted for the newly written data.
One approach to solving this problem has been to provide erase bands between consecutive data tracks. The head may have additional separate erase elements on either side of the write element in order to assure that the edge of each newly written field does not contain any data. Alternatively, the write element may be utilized to create the erase bands on either side of a newly written data field. Examples of this include the following references cited in U.S. Pat. No. 5,353,170 issued Oct. 4, 1994: U.S. Pat. No. 4,858,048 issued Aug. 15, 1989 and U.S. Pat. No. 4,771,346 issued Sep. 13, 1988.
MR heads (GMR and MRx included) generally contain separate read and write elements, with the write element being wider than the read element. The wide write element is a transducer element that is optimized for writing, while the narrower read MR element is optimized for reading. This MR recording head design permits smaller and more closely spaced data tracks to be written to and read from, thus increasing overall storage capacity of the HDA.
The relative narrowness of the read head with respect to the write head presents a problem related to incomplete data erasure. Because the read head is substantially narrower than the write head, an incompletely erased old data track may be mistakenly read as new data. The likelihood of reading an old data field in such a manner is further increased by the need to incrementally seek the track to on which the data is located. This track xe2x80x9chuntingxe2x80x9d only increases the chance that an old unerased data field at the edge of the track may be mistakenly read. The utility of erase bands on either side of the data track is limited by the close spacing between tracks. The erase bands must be sufficiently narrow such that the edge of a data track is erased without also erasing a portion of a neighboring track.
It would therefore be desirable to provide an improved data recovery method and system in which provides a safeguard against inadvertently reading old data within a HDA, such that I/O data reliability during data read operations can be ensured.
The above and other objects are achieved as is now described. A method and system are disclosed for adaptively limiting an off-track recovery displacement of a recording head during an off-track data recovery operation. A head disk assembly includes an actuator that positions the recording head in accordance with an actuator control signal. First, an off-track recover displacement threshold is determined utilizing an off-track data field which overlays an on-track data field, wherein the on-track data field has a distinctive data pattern with respect to the off-track data field. The recording head is displaced with respect to the track center until the on-track data field is read. The relative displacement required to retrieve the on-track data is utilized as the threshold. This threshold is stored in a reserved dedicated storage location on the disk from which it may be transferred to random access memory (RAM). In response to a failed read attempt, a data recovery procedure (DRP) commences whereby the recording head is incrementally displaced from the track center while attempting to recover the data. During the DRP, the predetermined off-track recovery threshold is utilized to limit the actuator displacement such that the input/output data integrity of the off-track recovery is ensured.